Limiting number of retransmission attempts for data transfer via network interface controller

ABSTRACT

An RNIC implementation that performs direct data placement to memory where all segments of a particular connection are aligned, or moves data through reassembly buffers where all segments of a particular connection are non-aligned. The type of connection that cuts-through without accessing the reassembly buffers is referred to as a “Fast” connection because it is highly likely to be aligned, while the other type is referred to as a “Slow” connection. When a consumer establishes a connection, it specifies a connection type. The connection type can change from Fast to Slow and back. The invention reduces memory bandwidth, latency, error recovery using TCP retransmit and provides for a “graceful recovery” from an empty receive queue. The implementation also may conduct CRC validation for a majority of inbound DDP segments in the Fast connection before sending a TCP acknowledgement (Ack) confirming segment reception.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates generally to data transfer, and moreparticularly, to an RDMA enabled network interface controller (RNIC)with a cut-through implementation for aligned DDP segments.

2. Related Art

1. Overview

Referring to FIG. 1A, a block diagram of a conventional data transferenvironment 1 is shown. Data transfer environment 1 includes a datasource 2 (i.e., a peer) that transmits a data transfer 3A via one ormore remote memory data access (RDMA) enabled network interfacecontroller(s) (RNIC) 4 to a data sink 5 (i.e., a peer) that receivesdata transfer 3B. RNIC 4 includes, inter alia (explained further below),reassembly buffers 6. Networking communication speeds have significantlyincreased recently from 10 mega bits per second (Mbps) through 100 Mbpsto 1 giga bits per second (Gbps), and are now approaching speeds in therange of 10 Gbps. The communications bandwidth increase, however, is nowbeginning to outpace the rate at which central processing units (CPUs)can process data efficiently, resulting in a bottleneck at serverprocessors, e.g., RNIC 4. For example, a common 1 Gbps networkconnection, if fully utilized, can be a large burden to a 2 GHz CPU. Inparticular, a CPU such as this can extend approximately half of itsprocessing power just handling low-level transmission control protocol(TCP) processing from data coming from a network card.

One approach to solving this problem has been to implement thetransmission control and Internet protocol (TCP/IP) stack in hardwarefinite state machines (FSM) rather than as software to be processed by aCPU. This approach allows for very fast packet processing resulting inwire speed processing of back-to-back short packets. In addition, thisapproach presents a very compact and powerful solution with low cost.Unfortunately, since the TCP/IP stack was defined and developed forimplementation in software, generating a TCP/IP stack in hardware hasresulted in a wide range of new problems. For example, problems thatarise include: how to implement a software-based protocol in hardwareFSMs and achieve improved performance, how to design an advantageous andefficient interface to upper layer protocols (ULPs) (e.g., applicationprotocols) to provide a faster implementation of the ULP, and how toavoid new bottle-necks in a scaled-up implementation.

In order to address these new problems, new communication layers havebeen developed to lay between the traditional ULP and the TCP/IP stack.Unfortunately, protocols placed over a TCP/IP stack typically requiremany copy operations because the ULP must supply buffers for indirectdata placement, which adds latency and consumes significant CPU andmemory resources. In order to reduce the amount of copy operations, asuite of new protocols, referred to as iWARP, have been developed.

2. The Protocols

Referring to FIG. 1B, a brief overview of various protocols, includingthe iWARP protocols, and data transfer format structure will now bedescribed. As can be seen, each data transfer may include informationrelated to a number of different protocols, each for providing differentfunctionality relative to the data transfer. For example, as shown inFIG. 1B, an Ethernet protocol 100 provides local area network (LAN)access as defined by IEEE standard 802.3; an Internet protocol (IP) 102adds necessary network routing information; a transfer control protocol(TCP) 104 schedules outbound TCP segments 106 and satisfies deliveryguarantees; and a marker with protocol data unit (PDU) alignment (MPA)protocol 108 provides an MPA frame 109 that includes a backward MPAmarker(s) 110 at a fixed interval (i.e., every 512 bytes) across DDPsegments 112 (only one shown, but may be stream) and also adds a lengthfield 114 and cyclic redundancy checking (CRC) field 116 to each MPAframe 109. In addition, a direct data placement (DDP) protocol 120segments outbound messages into one or more DDP segments 112, andreassembles one or more DDP segments into a DDP message 113; and aremote data memory access (RDMA) protocol 122 converts RDMA Write, Read,Sends into/out of DDP messages. Although only one DDP segment 112 hasbeen shown for clarity, it should be recognized that numerous DDPsegments 112 can be provided in each TCP segment 106.

With special regard to RDMA protocol 122, this protocol, developed bythe RDMA Consortium, enables removal of data copy operations andreduction in latencies by allowing one computer to directly placeinformation in another computer's memory with minimal demands on memorybus bandwidth and central processing unit (CPU) processing overhead,while preserving memory protection semantics. RDMA over TCP/IP promisesmore efficient and scalable computing and data transport within a datacenter by reducing the overhead burden on processors and memory, whichmakes processor resources available for other work, such as userapplications, and improves infrastructure utilization. In this case, asnetworks become more efficient, applications are better able to scale bysharing tasks across the network as opposed to centralizing work inlarger, more expensive systems. With RDMA functionality, a transmittercan use framing to put headers on Ethernet byte streams so that thosebyte streams can be more easily decoded and executed in an out-of-ordermode at the receiver, which will boost performance—especially forInternet Small Computer System Interface (iSCSI) and other storagetraffic types. Another advantage presented by RDMA is the ability toconverge functions in the data center over fewer types of interconnects.By converging functions over fewer interconnects, the resultinginfrastructure is less complex, easier to manage and provides theopportunity for architectural redundancy, which improves systemresiliency.

With special regard to the DDP protocol, this protocol introduces amechanism by which data may be placed directly into an upper layerprotocol's (ULP) receive buffer without intermediate buffers. DDPreduces, and in some cases eliminates, additional copying (to and fromreassembly buffers) performed by an RDMA enabled network interfacecontroller (RNIC) when processing inbound TCP segments.

3. Challenges

One challenge facing efficient implementation of TCP/IP with RDMA andDDP in a hardware setting is that standard TCP/IP off-load engine (TOE)implementations include reassembly buffers in receive logic to arrangeout-of-order received TCP streams, which increases copying operations.In addition, in order for direct data placement to the receiver's databuffers to be completed, the RNIC must be able to locate the destinationbuffer for each arriving TCP segment payload 127. As a result, all TCPsegments are saved to the reassembly buffers to ensure that they arein-order and the destination buffers can be located. In order to addressthis problem, iWARP specifications strongly recommend to thetransmitting RNIC to perform segmentation of RDMA messages in such waythat the created DDP segments would be “aligned” to TCP segments.Nonetheless, non-aligned DDP segments are oftentimes unavoidable,especially where the data transfer passes through many interchanges.

Referring to FIG. 1B, “alignment” means that a TCP header 126 isimmediately followed by a DDP segment 112 (i.e., MPA header follows TCPheader, then DDP header), and the DDP segment 112 is fully contained inthe one TCP segment 106. More specifically, each TCP segment 106includes a TCP header 126 and a TCP payload/TCP data 127. A “TCP hole”130 is a missing TCP segment(s) in the TCP data stream. MPA markers 110provide data for when an out-of-order TCP segment 106 is received, and areceiver wants to know whether MPA frame 109 inside TCP segment 106 isaligned or not with TCP segment 106. Each marker 110 is placed at equalintervals (512 bytes) in a TCP stream, starting with an Initial SequenceNumber of a particular connection, and points to a DDP/RDMA header 124of an MPA frame 109 that it travels in. A first sequentialidentification number is assigned to a first TCP segment 106, and eachInitial Sequence Number in subsequent TCP segments 106 includes anincremented sequence number.

In FIG. 1B, solid lines illustrate an example of an aligned datatransfer in which TCP header 126 is immediately followed by MPA lengthfield 114 and DDP/RDMA header 124, and DDP segment 112 is fullycontained in TCP segment 106. A dashed line in DDP protocol 120 layerindicates a non-aligned DDP segment 112NA in which TCP header 126 is notimmediately followed by MPA length field 114 and DDP/RDMA header 124. Anon-aligned DDP segment may result, for example, from re-segmentation bya middle-box that may stand in-between sending and receiving RNICs, or areduction of maximum segment size (MSS) on-the-fly. Since a transmitterRNIC cannot change DDP segmentation (change location of DDP headers inTCP stream), a retransmit operation may require a new, decreased MSSdespite the original DDP segments creation with a larger MSS. In anycase, the increase in copying operations reduces speed and efficiency.Accordingly, there is a need in the art for a way to handle aligned DDPsegment placement and delivery in a different fashion than non-alignedDDP segment placement and delivery.

Another challenge relative to non-aligned DDP segment 112NA handling iscreated by the fact that it is oftentimes difficult to determine what iscausing the non-alignment. For example, the single non-aligned DDPsegment 112NA can be split between two or more TCP segments 106 and oneof them may arrive and another may not arrive. In another case, some DDPsegments 112NA may fall between MPA markers 110, a header may bemissing, or a segment tail may be missing (in the latter case, you canpartially place the segment and need to keep some information tounderstand where to place the remaining part, when it arrives), etc.Relative to this latter case, FIG. 1C shows a block diagram of possiblesituations relative to MPA marker references for one or more non-alignedDDP segments 112NA. Case A illustrates a situation in which a DDPsegment header 160 of a newly received DDP segment 162 is referenced byan MPA length field 164 of a previously processed DDP segment 166. CaseB illustrates a situation in which newly received DDP segment 162 header160 is referenced by a marker 168 located inside newly received DDPsegment 162. That is, marker 168 is referring to the beginning of newlyreceived DDP segment 162. Case C illustrates a situation in which marker168 is located in newly received DDP segment 162, but points outside ofthe segment. Case D illustrates a situation in which marker 168 islocated in newly received DDP segment 162, and points inside thesegment. Case E illustrates a situation in which no marker is located innewly received DDP segment 162. In any case, where the cause of DDPsegment non-alignment cannot be determined, an RNIC cannot conductdirect data placement because there are too many cases to adequatelyaddress, and too much information/partial segments to hold in theintermediate storage. Accordingly, any solution that provides differenthandling of aligned and non-aligned DDP segments should address thevarious situations that may cause the non-alignment.

4. DDP/RDMA Operational Flow

Referring to FIGS. 1D-1H, a brief overview of DDP/RDMA operational flowwill now be described for purposes of later description. With specialregard to DDP protocol 120 (FIG. 1B), DDP provides two types of messagesreferred to as tagged and untagged messages. Referring to FIG. 1D, in a“tagged message,” each DDP segment 112 (FIG. 1B) carries a steering tag(“STag”) in DDP/RDMA header 124 that identifies a memory region/windowin a destination buffer (e.g., a memory region 232 in FIG. 1G) on areceiver to which data can be placed directly, a target offset (TO) inthis region/window and a segment payload (not shown). In this case,availability of the destination buffer is “advertised” via the STag.Referring to FIG. 1E, an “untagged message” is one in which a remotetransmitter does not know buffers at a receiver, and sends a messagewith a queue ID (QN), a message sequence number (MSN) and a messageoffset (MO), which may be used by the receiver to determine appropriatebuffers.

Referring to FIGS. 1F-1H, the RDMA protocol defines four types ofmessages: a Send 200, a Write 202, a Read 204, and a Read Response 206.Returning to FIG. 1A, a verb interface 7 presents RNIC 4 to a consumer,and includes methods to allocate and de-allocate RNIC 4 resources, andto post work requests (WR) 208 to RNIC 4. Verb interface 7 usually isimplemented by a verb library 8 having two parts: user space library 9Athat serves user space consumers and kernel module 9B that serves kernelspace consumers. Verb interface 7 is RNIC-specific software that workswith RNIC 4 hardware and firmware. There is no strict definition of whatshould be implemented in verb interface 7 (verb library 8), hardware andfirmware. Verb interface 7 can be viewed as a single package thatprovides RNIC 4 services to a consumer, so the consumer can performmainly two types of operations: management of RNIC 4 resources(allocation and de-allocation), and posting of work request(s) (WR) toRNIC 4. Examples of RNIC 4 resource management are: a queue pairallocation and de-allocation, a completion queue (hereinafter “CQ”)allocation and de-allocation or memory region allocation andde-allocation. These management tasks will be described in more detailbelow.

As shown in FIG. 1F-1H, a consumer allocates a queue pair to which workrequests 208 are posted. A “queue pair” (hereinafter “QP”) is associatedwith a TCP connection and includes a pair of work queues (e.g., send andreceive) 210, 212 as well as a posting mechanism (not shown) for eachqueue. Each work queue 210, 212 is a list of Work Queue Elements (WQE)216 where each WQE holds some control information describing one workrequest (WR) 208 and refers (or points) to the consumer buffers. Aconsumer posts a work request (WR) 208 to work queues 210, 212 in orderto get verb interface 7 (FIG. 1A) and RNIC 4 (FIG. 1A) to execute postedwork requests (WR) 208. In addition, there are resources that may makeup the QP with which the consumer does not directly interact such as aread queue 214 (FIG. 1H) and work queue elements (WQEs) 216.

The typical information that can be held by a WQE 216 is a consumer workrequest (WR) type (i.e., for a send WR 208S it can be RDMA Send, RDMAWrite, RDMA Read, etc., for a receive WR 208R it can be RDMA Receiveonly), and a description of consumer buffers that either carry data totransmit or represent a location for received data. A WQE 216 alwaysdescribes/corresponds to a single RDMA message. For example, when aconsumer posts a send work request (WR) 208S of the RDMA Write type,verb library 8 (FIG. 1A) builds a WQE 216S describing the consumerbuffers from which the data needs to be taken, and sent to theresponder, using an RDMA Write message. In another example, a receivework request (WR) 208R (FIG. 1F) is present. In this case, verb library8 (FIG. 1A) adds a WQE 216R to receive queue (RQ) 212 that holds aconsumer buffer that is to be used to place the payload of the receivedSend message 200.

When verb library 8 (FIG. 1A) adds a new WQE 216 to send queue (SQ) 210or receive queue (RQ) 212, it notifies (referred to herein as “ringsdoorbell”) of RNIC 4 (FIG. 1A) that a new WQE 216 has been added to sendqueue (SQ)/receive queue (RQ), respectively. This “doorbell ring”operation is usually a write to the RNIC memory space, which is detectedand decoded by RNIC hardware. Accordingly, a doorbell ring notifies theRNIC that there is new work that needs to the done for the specifiedSQ/RQ, respectively.

RNIC 4 (FIG. 1A) holds a list of send queues (SQs) 210 that have pending(posted) WQEs 216. In addition, the RNIC arbitrates between those sendqueues (SQs) 210, and serves them one after another. When RNIC 4 picks asend queue (SQ) 210 to serve, it reads the next WQE 216 to serve (WQEsare processed by the RNIC in the order they have been posted by aconsumer), and generates one or more DDP segments 220 belonging to therequested RDMA message.

Handling of the particular types of RDMA messages will now be describedwith reference to FIGS. 1F-1H. As shown in FIG. 1F, RNIC (Requester)selects to serve particular send queue (SQ) 210S. It reads WQE 216S fromsend queue (SQ) 210S. If this WQE 216S corresponds to an RDMA Sendrequest, RNIC generates a Send message, and sends this message to thepeer RNIC (Responder). The generated message may include, for example,three DDP segments 220. When RNIC (Responder) receives the Send message,it reads WQE 216R from receive queue (RQ) 212, and places the payload ofreceived DDP segments 220 to the consumer buffers (i.e. responder Rxbuff) 230 referred by that WQE 216R. If Send Message 200 is receivedin-order, then the RNIC picks the first unused WQE 216R from receivequeue (RQ) 212. WQEs 216R are chained in request queue (RQ) 212 in theorder they have been posted by a consumer. In terms of an untagged DDPmessage, Send message 200 carries a Message Sequence Number (MSN) (FIG.1E), which is initialized to one and monotonically increased by thetransmitter with each sent DDP message 220 belonging to the same DDPQueue. (Tagged messages will be described relative to RDMA Write message202 below). A DDP Queue is identified by Queue Number (QN) (FIG. 1E) inthe DDP header. The RDMA protocol defines three DDP Queues: QN #0 forinbound RDMA Sends, QN #1 for inbound RDMA Read Requests, and QN #2 forinbound Terminates. Accordingly, when Send message 200 arrivesout-of-order, RNIC 4 may use the MSN of that message to find the WQE216R that corresponds to that Send message 200. One received Sendmessage 200 consumes one WQE 216R from receive queue (RQ) 212. Lack of aposted WQE, or message data length exceeding the length of the WQEbuffers, is considered as a critical error and leads to connectiontermination.

Referring to FIGS. 1G and 1H, an RDMA Write message 202, using taggedoperations, and part of RDMA Read message 204 will now be described. Touse tagged operations, a consumer needs to register a memory region 232.Memory region 232 is a virtually contiguous chunk of pinned memory onthe receiver, i.e., responder in FIG. 1G. A memory region 232 isdescribed by its starting virtual address (VA), length, accesspermissions, and a list of physical pages associated with that memoryregion 232. As a result of memory region 232 registration, a consumerreceives back a steering tag (STag), which can be used to access thatregistered memory region 232. Access of memory region 232 by a remoteconsumer (e.g., requester in FIG. 1G) is performed by RNIC 4 without anyinteraction with the local consumer (e.g., responder in FIG. 1G). Whenthe consumer wants to access remote memory 232, it posts a send workrequest (WR) 208W or 208R (FIG. 1H) of the RDMA Write or RDMA Read type,respectively. Verb library 8 (FIG. 1A) adds corresponding WQEs 216W(FIG. 1G) or 216R (FIG. 1H) to send queue (SQ) 210W or 210R,respectively, and notifies RNIC 4. When connection wins arbitration,RNIC 16 reads WQEs 216W or 216R, and generates RDMA Write message 202 orRDMA Read message 204, respectively.

With special regard to RDMA Write message 202, as shown in FIG. 1G, whenan RDMA Write message 202 is received by RNIC 4, the RNIC uses the STagand TO (FIG. 1D) and length in the header of DDP segments (belonging tothat message) to find the registered memory region 232, and places thepayload of RDMA Write message 202 to memory 232. The receiver softwareor CPU (i.e., responder as shown) is not involved in the data placementoperation, and is not aware that this operation took place.

With special regard to an RDMA Read message 204, as shown in FIG. 1H,when the message is received by RNIC 4 (FIG. 1A), the RNIC generates aRDMA Read Response message 206, and sends it back to the remote host,i.e., requester as shown. In this case, the receive queue is referred toas a read queue 214. Generation of RDMA Read Response 206 is alsoperformed without involvement of the local consumer (i.e., responder),which is not aware that this operation took place. When the RDMA ReadResponse 206 is received, RNIC 4 (FIG. 1A) handles this messagesimilarly to handling an RDMA Write message 204. That is, it writes tomemory region 232 on the requester side.

In addition to handling consumer work requests, RNIC 4 (FIG. 1A) alsonotifies a consumer about completion of those requests, as shown inFIGS. 1F-1H. Completion notification is made by using completion queues240, another RNIC resource, which is allocated by a consumer (via adedicated function provided by verb library 8). A completion queue 240includes completion queue elements (CQE) 242. CQEs 242 are placed to acompletion queue (CQ) 240 by RNIC 4 (FIG. 1A) when it reports completionof a consumer work request (WR) 208S, 208W, 208RR. Each work queue(i.e., send queue (SQ) 210, receive queue (RQ) 212) has an associatedcompletion queue (CQ) 240. (Note: read queue 214 is an internal queuemaintained by hardware, and is invisible to software. Therefore, no CQ240 is associated with this queue, and the consumer does not allocatethis queue nor know about its existence). It should be noted, however,that the same completion queue (CQ) 240 can be associated with more thanone send queue (SQ) 210 and receive queue (RQ) 212. Association isperformed at queue pair (QP) allocation time. In operation, when aconsumer posts a work request WR 208 to a send queue (SQ) 210, it canspecify whether it wants to get a notification when this request iscompleted. If the consumer requested a completion notification, RNIC 4places a completion queue element (CQE) 242 to an associated completionqueue (CQ) 240 associated with send queue (SQ) 210 upon completion ofthe work request (WR). The RDMA protocol defines very simple completionordering for work requests (WR) 208 posted to a send queue (SQ) 210. Inparticular, RDMA send work requests (WR) 208S and RDMA write workrequests (WR) 208W are completed when they have been reliablytransmitted. An RDMA read work request (WR) 208R is completed when thecorresponding RDMA Read Response message 206 has been received, andplaced to memory region 232. Consumer work requests (WR) are completedin the order they are posted to send queue (SQ) 210. Referring to FIG.1F, each work request (WR) posted to a receive queue (RQ) 212 alsorequires completion notification. Therefore, when RNIC 4 (FIG. 1A)finishes placement of a received Send message 200, it places acompletion queue element (CQE) 242 to completion queue (CQ) 240associated with that receive queue (RQ) 212.

In view of the foregoing, there is a need in the art for a way to handlealigned DDP segment placement and delivery differently than non-alignedDDP segment placement and delivery.

SUMMARY OF THE INVENTION

The invention includes an RNIC implementation that performs direct dataplacement to memory where all received DDP segments of a particularconnection are aligned, or moves data through reassembly buffers wheresome DDP segments of a particular connection are non-aligned. The typeof connection that cuts-through without accessing the reassembly buffersis referred to as a “Fast” connection, while the other type is referredto as a “Slow” connection. When a consumer establishes a connection, itspecifies a connection type. For example, a connection that would gothrough the Internet to another continent has a low probability toarrive at a destination with aligned segments, and therefore should bespecified by a consumer as a “Slow” connection type. On the other hand,a connection that connects two servers in a storage area network (SAN)has a very high probability to have all DDP segments aligned, andtherefore would be specified by the consumer as a “Fast” connectiontype. The connection type can change from Fast to Slow and back. Theinvention reduces memory bandwidth, latency, error recovery using TCPretransmit and provides for a “graceful recovery” from an empty receivequeue, i.e., a case when the receive queue does not have a posted workqueue element (WQE) for an inbound untagged DDP segment. A conventionalimplementation would end with connection termination. In contrast, aFast connection according to the invention would drop such a segment,and use a TCP retransmit process to recover from this situation andavoid connection termination. The implementation also may conductcyclical redundancy checking (CRC) validation for a majority of inboundDDP segments in the Fast connection before sending a TCP acknowledgement(Ack) confirming segment reception. This allows efficient recovery usingTCP reliable services from data corruption detected by a CRC check.

A first aspect of the invention is directed to A method of limiting anumber of retransmission attempts for a data transfer via a networkinterface controller (NIC), the method comprising the steps of: storinga number of retransmission attempts for an error-including datatransfer; increasing the number of retransmission attempts each time aretransmission is attempted for the same error-including data transferpreviously received; and allowing another retransmission attempt of theerror-including data transfer in the case that the number ofretransmission attempts does not exceed a maximum retransmission attemptnumber.

A second aspect of the invention is directed to a system for limiting anumber of retransmission attempts for a data transfer via a networkinterface controller (NIC), the method comprising the steps of: meansfor storing a number of retransmission attempts for an error-includingdata transfer; means for increasing the number of retransmissionattempts each time a retransmission is attempted for the sameerror-including data transfer previously received; and means forallowing another transmission attempt of the error-including datatransfer in the case that the number of retransmission attempts does notexceed a maximum retransmission attempt number.

A third aspect of the invention is directed to a computer programproduct comprising a computer useable medium having computer readableprogram code embodied therein for limiting a number of retransmissionattempts for a data transfer via a network interface controller (NIC),the program product comprising: program code configured to store anumber of retransmission attempts for an error-including data transfer;program code configured to increase the number of retransmissionattempts each time a retransmission is attempted for the sameerror-including data transfer previously received; and program codeconfigured to allow another retransmission attempt of theerror-including data transfer in the case that the number ofretransmission attempts does not exceed a maximum retransmission attemptnumber.

The foregoing and other features of the invention will be apparent fromthe following more particular description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of this invention will be described in detail, withreference to the following figures, wherein like designations denotelike elements, and wherein:

FIG. 1A shows a block diagram of a conventional data transferenvironment and RNIC.

FIG. 1B shows a block diagram of conventional MPA/RDMA/DDP over TCP/IPdata transfer structure.

FIG. 1C shows a block diagram of possible MPA marker references for oneor more DDP segments.

FIG. 1D shows a block diagram of a conventional tagged DDP header.

FIG. 1E shows a block diagram of a conventional untagged DDP header.

FIGS. 1F-1H show block diagrams of various conventional RDMA messagedata transfers.

FIG. 2A shows a block diagram of a data transfer environment and RNICaccording to the invention.

FIG. 2B shows a block diagram of a connection context of the RNIC ofFIG. 2A.

FIG. 2C shows a block diagram of a validation unit of the RNIC of FIG.2A.

FIG. 3 shows a flow diagram of RNIC input logic (i.e., InLogic)functions.

FIGS. 4A-4B show flow diagrams for a limited retransmission attempt modeembodiment for the InLogic of FIG. 3.

FIG. 5 shows a block diagram illustrating handling of TCP segments afterconnection downgrading according to an alternative embodiment.

FIG. 6 shows a flow diagram for a connection upgrade embodiment for theInLogic of FIG. 3.

FIG. 7 shows an MPA request/reply frame for use with an initial sequencenumber negotiation implementation for cyclical redundancy checking (CRC)calculation and validation.

FIG. 8 shows a flow diagram for an alternative modified MPA lengthimplementation for CRC calculation and validation.

FIG. 9 shows a flow diagram for a first alternative embodiment ofInLogic using a no-markers cut-through implementation for CRCcalculation and validation.

FIG. 10 shows a flow diagram for a second alternative embodiment ofInLogic using the no-markers cut-through implementation for CRCcalculation and validation.

FIG. 11 shows a block diagram of RDMA Read and Read Response messagedata transfers including a Read Queue according to the invention.

FIG. 12 shows a block diagram of work queue elements (WQEs) and TCPholes for messages processed by RNIC output logic (i.e., OutLogic).

FIG. 13 shows a block diagram of RDMA Send message data transfersincluding a completion queue element (CQE) according to the invention.

FIG. 14 shows a block diagram of the CQE of FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

The following outline is provided for organizational purposes only: I.Overview, II. InLogic, III. OutLogic, and IV. Conclusion.

I. Overview

A. Environment

With reference to the accompanying drawings, FIG. 2A is a block diagramof data transfer environment 10 according to one embodiment of theinvention. Data transfer environment 10 includes a data source 12 (i.e.,a peer) that transmits a data transfer 14A via one or more remote memorydata access (RDMA) enabled network interface controller(s) (RNIC) 16 toa data sink 18 (i.e., a peer) that receives data transfer 14B. Forpurposes of description, an entity that initiates a data transfer willbe referred to herein as a “requester” and one that responds to the datatransfer will be referred to herein as a “responder.” Similarly, anentity that transmits data shall be referred to herein as a“transmitter,” and one that receives a data transfer will be referred toherein as a “receiver.” It should be recognized that each one of datasource 12 and sink 18 may, at different times, be a transmitter or areceiver of data or a requestor or a responder, and that the labels“source” and “sink” are provided only for purposes of initially denotingthat entity which holds the data to be transferred. The followingdescription may also refer to one of the above entities as a “consumer”(for its consuming of RNIC 16 resources), where a more specific label isnot necessary. “Destination buffers” shall refer to the data storagethat ultimately receives the data at a receiver, i.e., data buffers 50of data source 12 or data sink 18. Data source 12 and data sink 18 eachinclude data buffers 50 for storage of data.

In terms of hardware, RNIC 16 is any network interface controller suchas a network I/O adapter or embedded controller with iWARP and verbsfunctionality. RNIC 16 also includes a verb interface 20, an accesscontrol 30, RNIC input logic (hereinafter “InLogic”) 32, reassemblybuffers 34, an internal data buffer 38, RNIC output logic (hereinafter“OutLogic”) 40, a connection context 42, a validation unit 44 and othercomponents 46. Verb interface 20 is the presentation of RNIC 16 to aconsumer as implemented through the combination of RNIC 16 hardware andan RNIC driver (not shown) to perform operations. Verb interface 20includes a verb library 22 having two parts: a user space library 24 anda kernel module 26. Access control 30 may include any now known or laterdeveloped logic for controlling access to InLogic 32. Reassembly buffers34 may include any mechanism for temporary storage of data relative to adata transfer 14A, 14B. In particular, reassembly buffers 34 arecommonly used for temporary storage of out-of-order TCP streams, as willbe described in greater detail below. Other components 46 may includeany other logic, hardware, software, etc., necessary for operation ofRNIC 16, but not otherwise described herein.

Referring to FIG. 2B, connection context 42 includes a number of fieldsfor storing connection-specific data. Other context data 60 providesconnection-specific data not otherwise explained herein but recognizableto one having ordinary skill in the art. In accordance with theinvention, two connection types are defined: a Fast (hereinafter “FAST”)connection and a Slow (hereinafter “SLOW”) connection. The terms “Fast”and “Slow” refer to the connection's likelihood of delivering alignedDDP segments. The connection type is identified in a connection contextfield called ConnectionType 62. The SLOW connection may be used for RDMAconnections which either were created as SLOW connections, or weredowngraded by RNIC 16 during processing of inbound data, as will bedescribed in greater detail below. Other fields shown in FIG. 2B will bedescribed relative to their associated processing elsewhere in thisdisclosure. Referring to FIG. 2C, validation unit 44 includes cyclicredundancy checking (CRC) logic 64, TCP checksum logic 66 andstore-and-forward buffers 68 as may be necessary for validationprocessing.

B. RNIC General Operation

Returning to FIG. 2A, in operation, RNIC 16 receives data transfer 14Avia an access control 30 that controls access to InLogic 32. Informationfor sustaining the connection is retained in other context data 60 (FIG.2B) of connection context 42, as is conventional. InLogic 32 processesinbound TCP segments in data transfer 14A, performs validation ofreceived TCP segments via TCP checksum logic 66 (FIG. 2C), calculatesMPA CRC via CRC logic 64 (FIG. 2C), and separates FAST connection datastreams from SLOW connection data streams. With regard to the latterfunction, InLogic 32, as will be described more fully below, directs alldata received by RNIC 16 on a SLOW connection to reassembly buffers 34,and handles a FAST connection in a number of different ways. With regardto the FAST connections, if InLogic 32 detects an alignment violation(i.e., a TCP header is not immediately followed by a DDP Header, and theDDP segment is not fully contained in the one TCP segment), theconnection is downgraded to a SLOW connection and data is directed toreassembly buffers 34. In contrast, if an alignment violation is notpresent, InLogic 32 directs the aligned inbound DDP stream to aninternal data buffer 38 and then to OutLogic 40 for direct placement toa destination data buffer 50. Alternatively, a TCP segment 106 may bedropped, and no acknowledgement (Ack) sent, thus necessitating are-transmission of the segment.

OutLogic 40 arbitrates between FAST and SLOW connections, and performsdata placement of both connection type streams to data sink 18 databuffers 50. The situation in which aligned DDP segments on a FASTconnection are directed to internal data buffer 38 for direct placementto a destination buffer is referred to as the “cut-through mode” sinceFAST connections having aligned DDP segments are placed directly byOutLogic 40, bypassing reassembly buffer 34. For both connection types,however, only an in-order received data stream is delivered to data sink18 via OutLogic 40.

II. InLogic

With reference to FIG. 3, a flow diagram of InLogic 32 (FIG. 2A)according to the invention and its processing of a data transfer 14Awill be described in further detail. As noted above, InLogic 32processes inbound TCP segments, performs TCP validation of receivedsegments, calculates MPA CRC, and separates FAST connection data streamsfrom SLOW connection data streams. Unless otherwise noted, referencenumerals not followed by an “S” refer to structure shown in FIGS. 2A-2C.

In a first step S1, InLogic 32 filters TCP segments 106 of a datatransfer 14A belonging to RNIC 16 connections, and obtains packets withcalculated CRC validation (via validation unit 44) results for thereceived segments. (Note that CRC validation should be done beforeInLogic 32 decision processing. CRC validation can also be donesimultaneously with TCP checksum calculation, before TCP segment 106 isidentified as one belonging to a FAST connection—step S2.)

In step S2, InLogic 32 determines whether TCP segment 106 belongs to aSLOW connection. In this case, InLogic 32 determines how the transmitterlabeled the connection. If YES, TCP segment 106 is directed toreassembly buffers 34, and TCP logic considers this segment assuccessfully received, at step S3.

If NO, InLogic 32 proceeds, at step S4, to determine whether TCP segment106 length is greater than a stated MPA segment length. That is, whetherTCP segment 106 length, which is stated in TCP header 126, is longerthan an MPA length stated in MPA length field 114. If YES, thisindicates that TCP segment 106 includes multiple DDP segments 112, theprocessing of which will be described below. If NO, this indicates thatTCP segment 106 includes a single DDP segment 112 or 112NA.

In this latter case, at step S5, InLogic 32 determines whether the MPAlength is greater than TCP segment 106 length. If YES, this indicatesone of three situations: 1) the single DDP segment 112NA is not alignedto TCP segment 106, and the field that was assumed to be an MPA lengthfield is not a length field; 2) the beginning of the single DDP segment112 is aligned to TCP segment 106, but the length of the single DDPsegment exceeds TCP segment 106 payload size; or 3) the received singleDDP segment 112 is aligned to TCP segment 106, but has a corrupted MPAlength field 114. The first two cases (1 and 2) indicate that thenon-aligned single DDP segment 112NA has been received on a FASTconnection, and thus the connection should be downgraded to a SLOWconnection, at step S3. The third case (3) does not require connectiondowngrade. However, since the reason for MPA frame 109 length exceedingTCP segment 106 length cannot be identified and confirmed, the drop(i.e., cancellation and non-transfer) of such TCP segment 106 is notadvisable because it can lead to a deadlock (case 2, above). That is, ifsuch TCP segment indeed carried a non-aligned DDP segment, thetransmitter will retransmit the same non-aligned DDP segment, whichfollowing the same flow, would be repeatedly dropped by the receiverleading to a deadlock. Accordingly, InLogic 32, at step S3, directs datatransfer of TCP segment 106 to reassembly buffers 34, schedules an Ackto confirm that TCP segment 106 was successfully received, anddowngrades the connection to a SLOW connection (i.e., ConnectionTypefield 62 in FIG. 2B is switched from Fast to Slow). As will be describedbelow, if MPA length field 114 is corrupted (case 3 above), this isdetected by OutLogic 40, and the connection would be closed due to a CRCerror as detected by validation unit 44. Therefore, the connectiondowngrade, at step S3, would not cause the FAST connection topermanently become a SLOW connection due to data corruption in analigned DDP segment 112.

Returning to step S5, if MPA length is not greater than TCP length,i.e., NO, this indicates that MPA frame 109 length matches (equals) TCPsegment 106 length. InLogic 32 proceeds, at step S6, to determinewhether the CRC validation results are valid for this TCP segment 106.That is, whether CRC logic 64 returned a “valid” indication. If YES,this indicates that single DDP segment 112 exactly fits TCP segment 106boundaries (i.e., lengths are equal to one another), and no datacorruption has been detected for this segment. As a result, at step S7,single DDP segment 112 is processed in a “fast path mode” by placing thereceived TCP segment 106 to internal data buffer 38 of RNIC 16 forprocessing by OutLogic 40, which places the received TCP segment 106directly to the destination data buffers 50 of a receiver, e.g., of datasink 18. In addition, an Ack is scheduled to confirm successfulreception of this TCP segment 106.

If CRC logic 64 returns an “invalid” indication, i.e, NO at step S6,this indicates one of five possible cases exist that can be determinedaccording to the invention. FIG. 1C illustrates the five possible casesand steps S8-S10 illustrate how InLogic 32 handles each case. In anycase, the object of processing is to: 1) avoid termination ofnon-aligned connections, even if those were declared by a transmitter tobe a FAST connection; 2) reduce probability of connection terminationdue to data corruption in aligned DDP segments belonging to a FASTconnection; and 3) maintain InLogic 32 as simple as possible whilereducing the number of cases to be treated separately to a minimum.

At step S8, InLogic 32 determines, as shown as Case A in FIG. 1C,whether a DDP segment header 160 of a newly received DDP segment 162 isreferenced by an MPA length field 164 of a previously processed DDPsegment 166. In this case, the MPA length of previously processed DDPsegment 166 was checked during validation of MPA CRC of newly receivedDDP segment 162, and thus refers to the correct location of DDP header160 in the next segment. CRC invalidation for Case A, at step S6, meansthat the single DDP segment 162 data or header 160 has been corrupted.TCP retransmit of newly received segment 162 resolves this problem.Accordingly, at step S9, TCP segment 106 is dropped, and segmentreception is considered not confirmed.

If newly received DDP segment 162 header 160 is not referenced by MPAlength field 164 of previously processed DDP segment 166 (i.e., NO atstep S8), InLogic 32 proceeds, at step S10, to determine, as shown asCase B in FIG. 1C, whether newly received DDP segment 162 header 160 isreferenced by a marker 168 located inside newly received DDP segment162. That is, marker 168 is referring to the beginning of newly receivedDDP segment 162. In this case, CRC invalidation, at step S6, indicatesthat either: 1) marker 168 carries a correct value, and newly receivedDDP segment 162 has a corrupted DDP header 160 or data, or 2) marker 168inside newly received DDP segment 162 has been corrupted. In both casesretransmit of newly received DDP segment 162 resolves the problem.Accordingly, at step S9, the TCP segment is dropped, and segmentreception is not confirmed.

If newly received DDP segment 162 header 160 is not referenced by amarker 168 located inside newly received DDP segment 162, i.e., NO atstep S10, then one of three cases exist. First, as shown as Case C inFIG. 1C, marker 168 is located in newly received DDP segment 162, butpoints outside of the segment. Second, as shown as Case D in FIG. 1C,marker 168 is located in newly received DDP segment 162, but pointsinside the segment. Third, as shown as Case E in FIG. 1C, no marker islocated in newly received DDP segment 162.

In Cases C, D and E, the reason for CRC logic 64 returning an invalidindication is uncertain and can be the result of data corruption and/orreception of a non-aligned DDP segment 112NA (FIG. 1B). Unlimitedretransmit of such a segment can lead to deadlock in the case of anon-aligned DDP segment 112NA. To avoid potential deadlock, InLogic 32handles Cases C, D and E by, as shown at step S3, directing newlyreceived DDP segment 162 to reassembly buffers 34, scheduling an Ack toconfirm successful reception of the segment, and downgrading theconnection to a SLOW connection. If CRC logic 64 returning an invalidindication was due to data corruption in an aligned DDP segment 112,this error would be detected by OutLogic 40, as will be described below,when processing the data of the SLOW connection and the connection wouldbe terminated. Otherwise, the connection will remain a SLOW connectionforever. However, a Limited Retransmission Attempt Mode, as will bedescribed below, may prevent this problem.

Returning to step S4 of FIG. 3, if InLogic 32 determines that TCPsegment 106 length is greater than MPA frame 109 length this indicatesthat TCP segment 106 includes multiple DDP segments 112. In this case,at step S11, a sequential checking of CRC logic 64 validation results isconducted from a first to a last DDP segment 112. If all DDP segments112 have a valid CRC, i.e., YES, all DDP segments 112 are fullycontained in TCP segment 106, and all are valid, properly aligned DDPsegments 112. In this case, InLogic 32 processes DDP segments 112, atstep S7, on the fast path mode by placing the received TCP segment 106to internal data buffer 38 of RNIC 16 for processing by OutLogic 40,which places the received TCP segment 106 to the destination databuffers, e.g., data buffers 50 of data sink 18. In addition, an Ack isscheduled to confirm successful reception of this TCP segment 106.InLogic 32 stops checking CRC validation results when a first failurehas been detected, the management of which is explained relative tosteps S12-S13.

In step S12, InLogic 32 determines whether a first DDP segment 112 hasan invalid CRC as determined by CRC logic 64. If YES, InLogic 32processes the first DDP segment 112 similarly to an invalid CRC case fora single DDP segment (step S8). That is, InLogic 32 treats the first DDPsegment 112 with an invalid CRC as a single DDP segment 112 and proceedsto determine what caused the CRC invalidation, i.e., which of Cases A-Eof FIG. 1C applies, and how to appropriately handle the case.

If step S12 results in NO, i.e., the first DDP segment 112 has a validCRC, then InLogic 32 proceeds to determine whether CRC invalidity hasbeen detected when checking an intermediate or last DDP segment 112 atstep S13. If YES, InLogic 32 (FIG. 1) proceeds to step S9, since thiserror indicates that the data or header of DDP segment 112 that causedthe CRC invalidation has been corrupted (i.e., length of previous DDPsegment with valid CRC). That is, the CRC error was detected on theintermediate or last DDP segment 112 in the same TCP segment 106, whichmeans the preceding DDP segment has a valid CRC, and thus the length ofthe preceding DDP segment points to the header of the segment with theinvalid CRC. This matches the description of Case A (FIG. 1C).Therefore, as described in Case A, the location of the header is known,and therefore, the CRC error is known to have been caused either by dataor header corruption. Accordingly, a retransmit of the entire TCPsegment should resolve this problem, without any risk of the deadlockscenario. At step S9, the TCP segment is dropped, and segment receptionis not confirmed.

If step S13 results in NO, i.e., an intermediate or last DDP segment 112has not caused the CRC invalidation, then this indicates that MPA lengthfield 114 of the last DDP segment 112 exceeds TCP segment 106boundaries, i.e., the last DDP segment is outside of TCP segment 106boundaries or is too long. In this case, InLogic 32 treats the situationidentical to the single DDP segment 112 that is too long. In particular,InLogic 32 proceeds to, at step S3, direct data transfer 14A of TCPsegment 106 to reassembly buffers 34, schedules an Ack to confirm thatTCP segment 106 was successfully received, and downgrades the connectionto a SLOW connection. In this way, deadlock is avoided. If RNIC 16decides to drop one of the multiple DDP segments 112 contained in a TCPsegment 106, the entire TCP segment 106 is dropped, which simplifiesimplementation and reduces the number of cases that need to be handled.

Although not discussed explicitly above, it should be recognized thatother data transfer processing may also be carried in conjunction withthe above described operation of InLogic 32. For example, filtering ofTCP segments belonging to RNIC 16 connections and TCP/IP validations ofreceived segments may also be performed including checksum validationvia TCP checksum logic 66 (FIG. 2C). Processing of inbound TCP segment106 may also include calculation of MPA CRC, and validation of this CRCvia CRC logic 64 (FIG. 2C). One particular embodiment for CRCcalculation and validation will be further described below.

A. Limited Retransmission Attempt Mode

As an alternative embodiment relative to the uncertainty of the cause ofa detected error (e.g., NO at step S10 of FIG. 3 being one illustrativedetermination that may result in such a situation), a “limitedretransmission attempt mode” may be implemented to limit the number ofretransmit attempts to avoid deadlock and reduce the number of FASTconnections that are needlessly reduced to SLOW connections. Inparticular, as noted above, Cases C, D and E represent several cases inwhich, due to uncertainty of the cause of a detected error, theconnection may be downgraded to a SLOW connection (step S3) withpotential connection termination (by OutLogic 40) when the error wascaused by data corruption and not loss of DDP segment 112 alignment.

In order to limit the number of retransmit attempts, the presentinvention provides additional fields to connection context 42 (FIG. 2B)to allow for a certain number of retransmissions before downgrading theconnection. In particular, as shown in FIG. 2B, connection context 42includes a set of fields 290 including: a number of recovery attemptsfield (RecoveryAttemptsNum) 292, a last recovery sequence number field(LastRecoverySN) 294 and a maximum recovery attempts number field(MaxRecoveryAttemptsNum) 296. RecoveryAttemptsNum field 292 maintainsthe number of recovery attempts that were done for the connection sincethe last update; LastRecoverySN field 294 maintains a sequence number(SN) of the last initiated recovery operation; andMaxRecoveryAttemptsNum field 296 defines the maximum number of recoveryattempts that should be performed by InLogic 32 before downgrading theconnection.

Referring to FIG. 4A, in operation, when InLogic 32 detects that a newin-order received data transfer includes an error (shown generically asstep S101 in FIG. 4A), rather than immediately downgrade the connectionto a SLOW connection (at step S3 in FIG. 3), InLogic 32 provides for acertain number of retransmits to be conducted for that error-includingdata transfer. It should be recognized that step S101 is generic for anumber of error determinations (step S101 may apply, e.g., for a YES atstep S5 of FIG. 3 or a NO at step S10 of FIG. 3) that are caused eitherby a non-aligned DDP segment 112NA or a data corruption. At step S102,InLogic proceeds to record this transmission attempt for thiserror-including data transfer, step S1102, by increasingRecoveryAttemptsNum by one (1). In addition, InLogic updatesLastRecoverySN to store the largest sequence number between thepreviously stored sequence number therein and that of the newly received(but dropped) data transfer. That is, InLogic updates LastRecoverySN tostore the largest sequence number among at least one previously receivederror-including data transfer and the newly received error-including(but dropped) data transfer. The newly received error-including datatransfer is determined to have a sequence number greater than thelargest sequence number by comparing the sequence number of the newlyreceived error-including data transfer to the stored largest sequencenumber. The significance of LastRecoverySN recordation will becomeapparent below.

Next, at step S103, InLogic 32 determines whether theRecoveryAttemptsNum (field 292) exceeds the MaxRecoveryAttemptsNum(field 296). If NO, at step S104, InLogic 32 drops TCP segment 106 anddoes not confirm successful receipt, which causes a retransmission ofthe TCP segment. Processing then returns to step S1 (FIG. 3). If TCPsegment 106 was corrupted, then the retransmission should remedy thecorruption such that data transfer 14A is placed directly to memory as aFAST connection (at step S7 of FIG. 3). Alternatively, if processingcontinues to return other error detections (e.g., step S10 of FIG. 3),RecoveryAttemptsNum (field 292) will eventually exceedMaxRecoveryAttemptsNum (field 296) and result in a YES at step S106. Inthis case, InLogic 32 proceeds to step S105 at which InLogic 32downgrades the connection to a SLOW connection, places error-includingdata transfer 14A to reassembly buffer 34 and schedules an Ackconfirming successful reception of this TCP segment. The above processoccurs for each error-including data transfer.

FIG. 4B represents another component of the Limited RetransmissionAttempt Mode that addresses the fact that data corruption usually doesnot occur in multiple consecutive TCP segments, but non-aligned segmentsmay affect several subsequent TCP segments. For example, a FASTconnection may be sustained for a long period of time, e.g., five hours,and from time-to-time, e.g., once an hour, may have data corruption suchthat CRC validation will fail. As this occurs, the RecoveryAttemptsNum(field 292) may be increased each time the error-including data transfer(i.e., corrupted segment) is dropped. This process addresses thesituation where different segments are dropped due to data corruption atdifferent periods of time, and after several (probably one) retransmitoperation these segments are successfully received, and placed to thememory. Accordingly, the recovery operation for these segments wassuccessfully completed, and the data corruption cases that are recoveredfrom are not counted, i.e., when entering a new recovery mode due toreception of new errant segment.

In order to exit from the limited retransmission attempt mode, adetermination as to whether a TCP segment Sequence Number (SN) of anewly received in-order data transfer (i.e., InOrderTCPSegmentSN) isgreater than a LastRecovery Sequence Number (SN) (field 294 in FIG. 2B)is made at step S105. That is, a sequence number of each newly receivedin-order TCP segment belonging to a FAST connection is compared to astored largest sequence number selected from the one or more previouslyreceived error-including data transfers. (Note that reception of anout-of-order segment with larger SN does not mean that error recoverywas completed.) However, one indicator that recovery is complete is thata TCP segment is received that was transmitted after the segment(s) thatcaused entry to the recovery mode. This situation can be determined bycomparing the InOrderTCPSegmentSN with LastRecoverySN. Thisdetermination can be made at practically any stage of processing of theTCP segment received for this connection. For example, after step S9 inFIG. 3, or prior to step S102 in FIG. 4A. When the in-order segment SNis greater than the LastRecoverySN, i.e., a new TCP segment is received,and YES is determined at step S105, at step S106, RecoveryAttemptsNum(field 292 in FIG. 2B) is reset, i.e., set to zero. Relative to theabove example, step S105 prevents unnecessary downgrading of a FASTconnection to a SLOW connection after the long period of time, e.g.,five hours (i.e., because RecoveryAttemptsNum exceedsMaxRecoveryAttemptsNum), where the dropped segments were dropped due todata corruption and then, after the transmitter retransmitted thesegment, were successfully received and processed as an aligned segment.If NO at step S105 or after step S106, segment processing proceeds asusual, e.g., step S1 of FIG. 3.

Using the above processing, the number of retransmits allowed can beuser defined by setting MaxRecoveryAttemptsNum field 296. It should berecognized that while the limited retransmission attempt mode has beendescribed above relative to FIGS. 4A-4B and an error detection relativeto step S10 of FIG. 3, the limited retransmission attempt mode isapplicable beyond just the error detection of step S10, as will bedescribed further below. Note, that the limited retransmission attemptmode also finds advantageous use with part D, Speeding Up TCP RetransmitProcess, described below, which sends an immediate Duplicate Ack when asegment was dropped due to ULP considerations.

B. Connection Downgrading

Referring to FIG. 5, discussion of handling of a unique situation inwhich a connection is downgraded (step S3 in FIG. 3) after one or moreout-of-order received DDP segments 112 are placed to destination databuffers 50 in the fast path mode will now be described. As shown in FIG.5, four TCP segments labeled packet (Pkt) are received out-of-order,i.e., in the order 3, 4, 1 and 2. When a connection is downgraded to aSLOW connection, all data received from the moment of downgrading isplaced to reassembly buffers 34 and is reassembled to be in-order, i.e.,as Pkts 1, 2, 3 and 4. In this case, according to the TCP protocol,InLogic 32 maintains records that those segments were received.

Although rare, a situation may arise where a segment(s), e.g., Pkt #3(shaded), is/are directly placed to destination data buffers 50. Thissituation leads to the location in reassembly buffers 34 that wouldnormally hold packet 3 (Pkt# 3) being filled with ‘garbage’ data, i.e.,gaps or holes, even though InLogic 32 assumes that all data wasreceived. If processing is allowed to continue uncorrected, whenOutLogic 40 transfers reassembly buffers 34 to destination data buffers50, packet 3 (Pkt #3) that was earlier transferred on the fast path modewill be overwritten with the ‘garbage’ data, which will corrupt thedata.

To resolve this problem without adding hardware complexity, in analternative embodiment, InLogic 32 directs TCP logic to forget about thesegments that were out-of-order received when the connection was a FASTconnection (i.e., Pkt# 3 in FIG. 5). In particular, InLogic 32 isconfigured to clear a TCP hole for an out-of-order placed data transferwhen downgrading the connection to a SLOW connection at step S3 (FIG.3), and stops receipt reporting to the transmitter that these packetshave been received (SACK option). As a result, a transmitter retransmitsall not acknowledged data, including those segment(s) that wereout-of-order directly placed to destination data buffers 50, i.e., Pkt#3. When the retransmitted data is received, it is written to reassemblybuffers 34, and any out-of-order directly placed segments areoverwritten at destination data buffers 50 when OutLogic 40 transfersthe data from reassembly buffers 34. This functionality effectivelymeans that RNIC 16 ‘drops’ segments that were out-of-order placed todestination data buffers 50 in this connection. Such approach eliminatesthe case of ‘gapped’ in-order streams in reassembly buffers 34, and doesnot cause visible performance degradation because of the rare conditionsthat would lead to such behavior.

C. Connection Upgrade

As another alternative embodiment, the present invention may include aconnection upgrade procedure as illustrated in FIG. 6. The purpose ofthe fast path mode approach described above is to allow bypassing ofreassembly buffers 34 for a connection carrying aligned DDP segments112. However, even in FAST connections, a data source 12 or intermediatenetwork device can generate intermittent non-aligned DDP segments 112NA,which causes FAST connections to be downgraded to SLOW connectionsaccording to the above-described techniques. The intermittent behaviorcan be caused, for example, by maximum segment size (MSS) changes duringTCP retransmit, or other sporadic scenarios.

As shown in FIG. 6, to recover from this situation, the presentinvention may also provide a connection upgrade from a SLOW connectionto a FAST connection after an earlier downgrade, e.g., at step S3 (FIG.3). In order to accommodate the upgrade, a number of situations must bepresent. In a first step S31 of the alternative embodiment, InLogic 32determines whether reassembly buffers 34 are empty. If NO, then noupgrade occurs—step S32. If YES is determined at step S31, then at stepS33, InLogic 32 determines whether aligned DDP segments 112 are beingreceived. If NO, then no upgrade occurs—step S32. If YES is determinedat step S33, then at step S34, InLogic 32 determines whether theconnection was originated as a FAST connection by a transmitter, e.g.,data source 12. If NO is determined at step S24, then no upgradeoccurs—step S32. If YES is determined at step S34, the connection isupgraded to a FAST connection at step S35.

D. Speeding Up TCP Retransmit Process

Another alternative embodiment addresses the situation in which a TCPsegment 106 is received, but is dropped because of RDMA or ULPconsiderations, e.g., corruption, invalid CRC of DDP segments, etc.According to the above-described procedures, there are a number of timeswhere a TCP segment 106 is received and has passed TCP checksum, but isdropped by InLogic 32 without sending a TCP Ack covering the segment(i.e., step S9 of FIG. 3). Conventional procedures would then cause aretransmission attempt of those packets. In particular, in the basicscheme (the so-called “Reno protocol”), a TCP transmitter starts the‘Fast Retransmit’ mode when it gets three duplicated Acks (i.e., Acksthat do not advance the sequence number of in-order received data). Forexample, assume two TCP segments A and B, and that segment B followssegment A in TCP order. If segment A is dropped, then the receiver wouldsend a duplicate Ack only when it receives segment B. This duplicate Ackwould indicate “I'm waiting for segment A, but received anothersegment,” i.e., segment B. In the ‘Fast Retransmit’ mode under the Renoprotocol, the transmitter sends one segment, then it waits for anotherthree duplicate Acks to retransmit another packet. More advanced schemes(like the so-called “New-Reno protocol”) allow retransmitting of asegment for each received duplicate in its ‘Fast Recovery’ mode. Thelogic behind this process being that if one segment left the network,then the transmitter may put another packet to the network.

In order to facilitate re-transmission, according to an alternativeembodiment of the invention, InLogic 32 generates a first duplicate TCPacknowledgement (Ack) covering a received TCP segment that is determinedto be valid by TCP and was dropped by TCP based on an upper layerprotocol (ULP) decision (e.g., at step S9 of FIG. 3); and transmits theduplicate TCP Ack. The ULP, as noted above, may include one or more of:an MPA protocol, a DDP protocol, and a RDMA protocol. The firstduplicate TCP Ack is generated for a TCP segment regardless of whetherthe TCP segment is in-order or out-of-order, and even where a nextin-order TCP segment has not been received. InLogic 32 may also generatea second duplicate TCP acknowledgement (Ack) covering a nextout-of-order received TCP segment, and transmit the second duplicate TCPAck.

This above processing effectively means generation of a duplicate Ack(e.g., for segment A in example above) even though the next in-ordersegment (e.g., segment B in example above) may not have been receivedyet, and thus should speed up a process of re-entering the transmitterto the fast path mode under the above-described retransmission rules.More specifically, even if segment B has not been received, thetransmitter would know that segment A, a valid TCP segment, was receivedand dropped due to ULP considerations. As a result, the additionalduplicate Ack forces the transmitter to begin the retransmit procedureearlier where a number of duplicate Acks must be received beforeretransmission begins. This approach does not violate TCP principles,since TCP segment 106 has been successfully delivered to the ULP, anddropped due to ULP considerations (invalid CRC). Therefore the packetwas not dropped or reordered by the IP protocol. This approach isparticularly valuable when RNIC 16 implements the limited retransmissionattempt mode as outlined relative to FIG. 4A, i.e., an Ack is sent atstep S103.

E. CRC Calculation and Validation

Conventional processing of incoming Ethernet frames starts with afiltering process. The purpose of filtering is to separate validEthernet frames from invalid ones. “Invalid frames” are not corruptedframes, but frames that should not be received by RNIC 16, e.g., MACfiltering-frame selection based on MAC addresses, virtual local areanetwork (VLAN) filtering—frame selection based on VLAD Tags, etc. Thevalid frames, that were allowed to get into RNIC 16, are also separatedinto different types. One of these types is a TCP segment. The filteringprocess is done on the fly, without any need to performstore-and-forward processing of the entire Ethernet frame.

The next step of TCP segment processing is TCP checksum calculation andvalidation. Checksum calculation determines whether data was transmittedwithout error by calculating a value at transmission, normally using thebinary values in a block of data, using some algorithm and storing theresults with the data for comparison with the value calculated in thesame manner upon receipt. Checksum calculation and validation requiresstore-and-forward processing of an entire TCP segment because it coversan entire TCP segment payload. Conventionally, calculation andvalidation of cyclical redundancy checking (CRC) normally follows TCPchecksum validation, i.e., after a connection is recognized as an RDMAconnection and after the boundaries of a DDP segment have been detectedeither using a length of a previous DDP segment or MPA markers. CRCcalculation and validation determines whether data has been transmittedaccurately by dividing the messages into predetermined lengths which,used as dividends, are divided by a fixed divisor. The remainder of thecalculation is appended to the message for comparison with an identicalcalculation conducted by the receiver. CRC calculation and validationalso requires store-and-forward of an entire DDP segment, whichincreases latency and requires large data buffers for storage. Onerequirement of CRC calculation is to know DDP segment boundaries, whichare determined either using the length of the preceding DDP segment orusing MPA markers 110 (FIG. 1B). The marker-based determination is verycomplicated due to the many exceptions and corner cases. CRC calculationof a partially received DDP segment is also a complicated process.

In order to address the above problems, as shown in FIG. 2C, the presentinvention performs CRC calculation and validation via CRC logic 64 inparallel with TCP checksum calculation and validation via TCP checksumlogic 66 using the same store-and-forward buffer 68. In addition, thepresent invention does not immediately locate DDP segment boundaries,and then calculate and validate DDP segment CRC. Rather, the presentinvention switches the order of operations by calculating CRC and laterdetermining DDP boundaries. In order to make this switch, CRC logic 64assumes that each TCP segment (before it is known that the segmentbelongs to an RDMA connection) starts with an aligned DDP segment. Inaddition, the present invention assumes that the first two bytes of aTCP payload 127 (FIG. 1B) is an MPA length field 114 (FIG. 1B) of an MPAframe. This length is then used to identify the DDP segment boundariesand calculate CRC for that segment. After validation unit 44 identifiesa boundary of the first possible DDP segment 112 in TCP segment 106, itcalculates and validates CRC for that DDP segment simultaneously withthe checksum calculation for that portion of TCP segment payload 127,and then proceeds to the next potential DDP segment 112 (if any)contained in the same TCP segment 106. For each “potential” DDP segmentdiscovered in TCP segment 106, CRC validation results may be valid,invalid or too long. Results of CRC validation are stored for use asdescribed above relative to FIG. 3.

In order to actually calculate CRC as described above, when the payloadof a TCP segment 106 is processed, InLogic 32 needs to know where MPAmarkers 110 are in a TCP segment 106. As discussed above relative toFIG. 1B, MPA markers 110 are placed every 512 bytes apart in a TCPsegment 106, and the first MPA marker is 512 bytes from an InitialSequence Number in TCP header 126 (FIG. 1B), which is stored as StartNumfield 248 (FIG. 2B) of connection context 42. Unfortunately, anevaluation of each MPA marker 110 does not reveal its position relativeto StartNum 248 (FIG. 2B). In addition, MPA markers 110 are covered byCRC data 116, but are not included in an MPA length field 114, whichincludes only the payload of an MPA frame. Accordingly, to identify MPAmarkers 110, RNIC 16 needs to know StartNum 248 (FIG. 2B), which must befetched from connection context 42. Unfortunately, reading connectioncontext 42 is very inconvenient to conduct during TCP processing as itoccurs very early in processing and breaks up or holds up packetprocessing.

In order to reduce or eliminate connection context 42 fetching, thepresent invention presents four alternatives allowing correctcalculation of DDP segment 112 length, which is required to calculateand validate MPA CRC of that segment. These options are discussed in thefollowing sections.

1. Connection Context Prefetch Method

A first alternative embodiment for correctly calculating DDP segment 112length includes implementing a connection context 42 prefetch of anInitial Sequence Number stored as StartNum field 248 (FIG. 2B). Nochange to the MPA specification is proposed here. The current MPAspecification requires knowledge of an Initial Sequence Number(StartNum) to identify the location of an MPA marker 110 in a TCPsegment 106. The Initial Sequence Number is a TCP connection attribute,which varies from connection to connection and is negotiated atconnection establishment time. Therefore, a StartNum 248 (FIG. 2B) ismaintained on a per connection basis. To identify the location of MPAmarker 110, CRC logic 64 (FIG. 2C) checks that the remainder of aparticular segment's sequence number (SeqNum) and StartNum(SeqNum-StartNum) mod 512 is zero. That is, because each TCP segment 106header carries the sequence number of the first byte of its payload, CRClogic 64 can determine where to look for a marker by taking a differencebetween the particular segment's sequence number and StartNum248, andthen starting from this position, locate a marker every 512 bytes. TheMPA specification defines the above-described marker detection method.In this way, a Hash lookup (based on TCP tuple) and a connection context42 prefetch can be performed before the TCP checksum validation isperformed. This is a normal connection context 42 fetch flow. If RNIC 16wants to get connection context 42, it first needs to understand wherethis context is located, or get the Connection ID. TCP segment 106header carries TCP tuple (IP addresses (source and destination) and TCPports (source and destination)). Tuple is an input to Hash function. Theoutput of Hash function is a Connection ID. Of course, the sameConnection ID for different tuples may result, which is called“collision.” To handle collisions, RNIC 16 reads connection context 42,checks the tuple in connection context 42 with the tuple in the packet,and if it does not match, then RNIC 16 gets the pointer to the nextconnection context 42. RNIC 16 keeps checking tuples until it eitherfinds the match, or the segment is recognized as one that does notbelong to any known connection. This process allows locating MPA markers110 in TCP stream. As a result, CRC calculation and validation can beperformed simultaneously with TCP checksum validation.

2. Initial Sequence Number Negotiation Method

In a second alternative embodiment, correctly calculating DDP segmentlength is possible without connection context fetching by making anumber of changes to the MPA specification. First, the definition of MPAmarker 110 placement in the MPA specification is changed. Onedisadvantage of the above-described Connection Context Prefetch Methodis the need to perform a Hash lookup and connection context 42 prefetchto identify boundaries of the MPA frame 109 in a TCP segment 106. Inorder to prevent this, the present invention places MPA markers 110every 512 bytes rather than every 512 bytes starting with the InitialSequence Number (SN)(saved as StartNum 248) (which necessitates theabove-described SN-StartNum mod 512 processing). In this fashion, MPAmarkers 110 location may be determined by a sequence number mod 512process to locate MPA markers 110, and no connection context 42 fetch isrequired.

A second change to the MPA specification according to this embodimentacts to avoid the situation where one marker is split between two DDPsegments 112, i.e., where an Initial Sequence Number is notword-aligned. As a result, a sequence number mod 512 process may notwork in all circumstances because the standard TCP implementation allowsthe Initial SN to have a randomly generated byte-aligned value. That is,whether an Initial Sequence Number is word-aligned is not controllableby RNIC 16. As a result, a TCP stream for the given connection may notnecessarily start with an MPA marker 110. Accordingly, if CRC logic 64picks the location of a marker 110 just by using the sequence number mod512 process, it could get markers placed to the byte aligned location,which is unacceptable. To avoid this situation, the present inventionadds padding to MPA frames exchanged during an MPA negotiation stage,i.e., the so called “MPA request/reply frame,” to make the Initial SN ofan RDMA connection when it moves to RDMA mode, word-aligned. That is, asshown in FIG. 7, a correction factor 150 is inserted into an MPArequest/reply frame 152 of a TCP segment 106 that includes the number ofbytes needed to make the Initial SN word-aligned. It should berecognized that the exact location of correction factor 150 does nothave to be as shown. In this way, CRC logic 64 may implement thesequence number mod 512 process to obtain the exact location of the MPAmarkers 110 in TCP stream without a connection context fetch. Using theabove-described modifications of the MPA specification, the inventioncan locate MPA markers 110 and properly calculate the length of MPAsegment without prefetching connection context 42.

3. MPA Length Field Modification Method

In a third alternative embodiment for correctly calculating DDP segment112 length without connection context fetching, a definition of MPAlength field 114 is changed in the MPA specification. Conventionally,MPA length field 114 is defined to carry the length of the ULP payloadof a respective MPA frame 109, excluding markers 110, padding 121 (FIG.1B) and CRC data 116 added by the MPA layer. Unfortunately, thisinformation does not allow locating of MPA frame boundaries usinginformation provided by TCP segment 106. In order to address this,according to this alternative embodiment, the definition of MPA lengthin the MPA specification is changed to specify a length of the entireMPA frame 109 including: 14 most-significant bits (MSBs) of MPA lengthfield 114, ULP payload 118 length, MPA markers 110, CRC data 116, 2least-significant bits (LSBs) of MPA length field 114, and valid bits inpadding 121.

This revised definition allows detection of MPA frame 109 boundariesusing MPA length field 114 without locating all MPA Markers 110 embeddedin that MPA frame. MPA layer protocol is responsible for strippingmarkers 110, CRC data 116 and padding 121 and provide the ULP (DDPLayer) with ULP payload length.

Referring to FIG. 8, using this definition of MPA length, CRC logic 64locates the boundaries of MPA frame 109 by the following process: Instep S1100, CRC logic 64 determines whether the first word of an MPAframe 109 equals zero. If YES, then InLogic 32 (FIG. 2A) reads MPAlength field 114 from the next word at step S102. This is the case whena marker 110 falls between two MPA frames 109. In this situation, MPAlength field 114 is located in the next word as indicated at step S104.If NO is the determination at step S100, then this word holds MPA lengthfield 114. In step S106, the MPA length is used to find the location ofthe CRC data 116 covering this MPA frame 109. The above process thenrepeats to locate other MPA frames 109 embedded in TCP segment 106. Thisembodiment allows locating of MPA frame 109 boundaries without anyadditional information from connection context 42.

4. No-Markers Cut-Through Implementation

In a fourth alternative embodiment, a no-marker cut-throughimplementation is used relative to CRC calculation and validation, aswill be described below. A disadvantage of the above-described threealternative embodiments for correctly calculating DDP segment length isthat each requires modification of the MPA specification or connectioncontext 42 prefetching. This embodiment implements a cut-throughprocessing of inbound segments without prefetching connection context 42to calculate CRC of arriving MPA frames and without any additionalchanges to the MPA specification. In addition, this embodiment allowsout-of-order direct data placement without use of MPA Markers. Thisembodiment is based, in part, on the ability of a receiver to negotiatea ‘no-markers’ option for a given connection according to a recentupdated version of the MPA specification. In particular, the updated MPAspecification allows an MPA receiver to decide whether to use markers ornot for a given connection, and the sender must respect the receiver'sdecision. This embodiment changes validation unit 44 logic to allow CRCcalculation on the fly concurrently with TCP checksum calculation andwithout prefetching connection context 42.

The CRC calculation is done exactly as described for the case withmarkers. That is, the present invention assumes that the TCP segmentstarts with aligned DDP segment, and uses the MPA length field to findthe location of CRC, and then calculates and validates CRC. Thedifference with this embodiment, however, is that there is no need toconsider markers when calculating DDP segment length, given MPA lengthfield of the MPA header.

Referring to FIG. 9, a flow diagram illustrating InLogic 32functionality relative to a first alternative of this embodiment isshown. It should be recognized that much of InLogic 32 functionality issubstantially similar to that described above relative to FIG. 3. Forclarity purposes, where InLogic 32 functionality is substantiallysimilar to that described above relative to FIG. 3, the steps have beenrepeated and delineated with a dashed box.

Under the updated MPA specification, a receiver negotiates a ‘no-marker’option for a particular connection at connection initialization time. Asshown in FIG. 9, in this embodiment, at step S201, InLogic 32 determineswhether inbound TCP segment 106 includes markers 110. If YES, InLogic 32proceeds with processing as in FIG. 3, and some other method of CRCcalculation and validation would be used, as described above. If NO, atstep S202, inbound MPA frames 109 have their CRC calculated andvalidated on the fly using the same store-and-forward buffers 68 as TCPchecksum logic 66, but without fetching connection context 42. Adetermination of whether the connection is a SLOW connection, steps S2and S3 as in FIG. 3, may also be completed. Results of CRC validationcan be one of the following: 1) the length of MPA frame 109 matches thelength of TCP segment 106, and MPA frame 109 has a valid MPA CRC; 2) thelength of the MPA frame 109 matches the length of TCP segment 106, butMPA frame 109 has an invalid CRC; 3) the length of MPA frame 109 exceedsthe length of the TCP segment; and 4) the length of MPA frame 109 issmaller than the length of TCP segment 106.

In case 1), InLogic 32 functions substantially similar to steps S4-S7 ofFIG. 3. That is, where MPA frame 109 has a same length as a TCP segment106 (steps S4 and S5 of FIG. 3), and carries a valid MPA CRC (step S6),the frame is considered to be a valid MPA frame, and is passed toOutLogic 40 for further processing via internal data buffers 38 and todestination data buffers 50 on the fast path mode.

In case 2), where MPA frame 109 has a same length as a TCP segment 106(steps S4 and S5 of FIG. 3), but has an invalid CRC (step S6 of FIG. 3),InLogic 32 functions differently than described relative to FIG. 3. Inparticular, since received MPA frame 109 does not contain MPA markers110, the marker related information cannot be used for recovery (as instep S10 of FIG. 3). This leaves only two cases that need to beaddressed: Case A: when MPA frame 109 is referred by the length of thepreviously received segment (and validated) MPA frame 109 (as determinedat step S8 of FIG. 3); and Case B: all other cases. In Case A the MPAframe 109 is corrupted, and in Case B, MPA frame 109 can be eithercorrupted or not aligned. In both cases the received TCP segment 106 isdropped (step S9 of FIG. 3), and receipt is not confirmed. In this case,the limited retransmission attempt mode described relative to FIG. 4 maybe implemented to recover from the drop of that TCP segment 106, whichallows the sender to retransmit the dropped TCP segment 106 and resolveany potential data corruption. If MPA frame 109 was not aligned to TCPsegment 106, then the limited retransmission attempt mode will end withdowngrading of the connection to a SLOW connection, as described above.

In case 3), where the length of MPA frame 109 exceeds a length of TCPsegment 106 (step S5 of FIG. 3), either MPA frame 109 is not aligned toTCP segment 106, or the length is corrupted. In this case, the receivedTCP segment 106 is dropped (step S9 of FIG. 3), and TCP does not confirmreceipt. In this case, again, the limited retransmission attempt modedescribed relative to FIG. 4 may be implemented to recover from the dropof that TCP segment 106, which allows the sender to retransmit thedropped TCP segment and resolve any potential data corruption. Again, ifMPA frame 109 is not aligned to TCP segment 106, then the limitedretransmission attempt mode will end with downgrading of the connectionto a SLOW connection, as described above.

In case 4), where the length of MPA frame 109 is smaller than the lengthof TCP segment 106 (step S4 of FIG. 3), or TCP segment 106 potentiallycarries multiple MPA frames 109 (sender exercises a packing option),InLogic 32 sequentially checks the CRCs of all DDP segments 112 embeddedin the received TCP segment 106 (steps S11-S13 of FIG. 3). If all DDPsegments 112 have a valid CRC, InLogic 32 approves reception of that TCPsegment 106, and all MPA frames are forwarded for the further processingon the fast path mode (step S7 of FIG. 3). If one of DDP segments 112has an invalid CRC, or the last segment is not fully contained in theTCP segment (steps S12-S13 of FIG. 3), the entire TCP segment is dropped(step S9 of FIG. 3), and InLogic 32 does not confirm reception of thatTCP segment. As above, the limited retransmission attempt mode describedrelative to FIG. 4 may be implemented to recover from the drop of thatTCP segment 106, which allows the sender to retransmit the dropped TCPsegment and resolve any potential data corruption. If MPA frame 109 wasnot aligned to TCP segment 106, then the limited retransmission attemptmode will end with downgrading of the connection to a SLOW connection,as described above.

Turning to FIG. 10, another alternative flow diagram illustratingInLogic 32 functionality relative to this embodiment, and includingaspects of the Limited Retransmission Attempt Mode and TCP RetransmitSpeed-Up is shown. In contrast to FIG. 9, InLogic 32 functionality isgreatly simplified compared to FIG. 3. For clarity purposes, whereInLogic 32 functionality is substantially similar to that describedabove relative to FIG. 3, the steps have been repeated and delineatedwith a dashed box.

In FIG. 10, steps S151-S153 are substantially identical to step S1-S3 ofFIG. 3. At step S154, InLogic 32 determines whether CRC validationpassed. This evaluation is different than step S4 in FIG. 3 in thatinstead of providing an indication per DDP segment, CRC logic 54provides a CRCValidationPassed bit that indicates success or failure ofCRC validation of all DDP segments in a received TCP segment. This bitis set if the CRC validation passed for all DDP segments contained inreceived TCP segment, and is cleared if either the CRC validation failedfor one of the segments, or the last (only) segment was too long. If NO,InLogic 32 proceeds to step S155, where a determination as to whetherRecoveryAttemptsNum (field 292 of FIG. 2B) is greater thanMaxRecoveryAttemptsNum (field 296 of FIG. 2B). If YES, then InLogicproceeds to step S153 where the DDP segment is placed to reassemblybuffers 34, an Ack is sent, and the connection is downgraded to a SLOWconnection (if it was a FAST connection). If NO at step S155, then atstep S156, the TCP segment 106 is dropped and no confirmation isscheduled. In addition, RecoveryAttemptNum (field 292 of FIG. 2B) isincreased by one, and the LastRecoverySN (field 294 of FIG. 2B) isupdated.

Returning to step S154, if the determination results in a YES, InLogic32 proceeds, at step S157, to determine whether a newly receivedin-order data transfer's sequence number (In-order SN) is greater thanLastRecoverySN (field 294 of FIG. 11B). If YES, then at step S158,InLogic 32 clears RecoveryAttemptsNum (field 292 in FIG. 1B), i.e., setsit to zero. If NO at step S157 or subsequent to step S158, at step S159,the segment is processed on the “fast path mode” by placing the segmentto destination data buffers 50. Step S159 may also includeimplementation of the duplicate Ack, as discussed above relative to theTCP Retransmit Speed-Up option.

The above-described FIG. 10 embodiment implements the cut-through modeof the invention plus the limited retransmission attempt mode and TCPretransmit speed-up option without use of MPA markers.

III. OutLogic

OutLogic 40 (FIG. 2A) performs in-order delivery of RDMA messageswithout keeping information per RDMA message. There are two situationsthat are addressed: 1) for all RDMA Messages excepting a Send message,and 2) an RDMA Send message.

Returning to FIGS. 1F-1H, operation of OutLogic 40 (FIG. 2A) will now bedescribed. OutLogic processes aligned DDP segments 220 from internaldata buffers 38 (FIG. 2A) that were placed there on the fast path mode,as described above, and conducts data placement and delivery of thealigned DDP segments to a receiver's data buffers. As used herein,“placement” refers to the process of actually putting data in a buffer,and “delivery” refers to the process of confirming completion of a datatransfer. “Placement” may be applied to both segments and messages,while “delivery” applies to messages only. Under the RDMA protocol,aligned DDP segments may be placed in an out-of-order fashion, butdelivery does not occur until all of the aligned DDP segments are placedin-order. For example, for three aligned DDP segments 1, 2 and 3, wheresegments 2 and 3 are first placed without segment 1, delivery does notoccur until segment 1 is placed.

A. Placement

With regard to placement, OutLogic 40 provides conventional placement ofRDMA messages except relative to RDMA Read messages, as will bedescribed below.

With regard to tagged DDP segments, for example, returning to FIG. 1D,according to the RDMA protocol, a header 124 of a tagged DDP segmentcarries an address of the receiver's previously registered memory region(e.g, memory region 232 in FIG. 1G). As indicated above, this addressincludes starting tag (STag) indicating a destination buffer that liesin memory region/window (e.g., memory region 232 in FIG. 1G for an RDMAWrite message), a target offset (TO) in this region/window and atransaction length (segment payload). In this case, data placement isconducted by OutLogic 40 in a conventional manner, without retrievingany additional information from connection context 42 (FIG. 2A).Conventional Address Translation and Protection (ATP) processes, inwhich the STag and TO are translated to a list of physical buffers of amemory region describing the destination data buffer, precedes the dataplacement by OutLogic 40.

Relative to untagged DDP segments such as an RDMA Read message,referring to FIG. 1H, the RDMA protocol defines the maximal number ofpending inbound Read Requests 222, which is exchanged at negotiationtime. Each RDMA Read message 204 consumes a single DDP segment 222. WhenRNIC 16 receives RDMA Read message 204, it posts an RDMA Read ResponseWQE 216RR to a Read Queue 214. In another example, referring to FIG. 1F,each Send message 200 is placed to receive queue (RQ) 212 of aresponder, e.g., data sink 18 (FIG. 2A). As noted above, each receivequeue (RQ) 212 is a buffer to which control instructions are placed, andincludes a WQE 216R to which a payload is placed. Receive queue (RQ) 212includes WQEs 216R. Each WQE 216R holds control information describing areceive WR 208R posted by a consumer. Each WQE 216R also points onconsumer buffer(s) posted in that WR 208R. Those buffers are used toplace the payload. Accordingly, each message 200 consumes a WQE 216R.

Referring to FIG. 11, a representation of an RDMA Read message 204 andRDMA Read Response 206 similar to FIG. 1H is shown. In accordance withthe invention, however, a Read Queue 414 is provided as a special workqueue (WQ) implemented as a cyclic buffer, and each entry of this cyclicbuffer is a WQE 216RR describing the RDMA Read Response that needs to begenerated by transmit logic. This allows easy and efficient placement ofout-of-order RDMA Read Requests 222 since for each inbound RDMA ReadRequest there is a well known location in the Read Queue 414, i.e., WQE216RR. For example, when RDMA Read message #3 is received and RDMA Readmessage #2 is lost, RDMA Read message #3 is placed. This placement isdone upon reception of RDMA Read Request message 222, i.e., message sentdue to posting of Read WR 208R on requester. Location of WQE 216RR inRead Queue 414 is identified by the MSN in RDMA Read message header 124(FIG. 1D).

B. Delivery

The RDMA protocol allows out-of-order data placement but requiresin-order delivery. Accordingly, conventional implementations requiremaintaining information about each message that was placed (fully orpartially) to the memory, but not delivered yet. Loss of a single TCPsegment, however, can lead to the reception of many out-of-order RDMAmessages, which would be placed to the destination buffers, and notcompleted until the missing segment would be retransmitted, andsuccessfully placed to the memory. Under conventional circumstances,limited resources are available to store an out-of-order stream suchthat only a certain number of subsequent messages can be stored after anout-of-order stream is received.

According to the invention, however, instead of holding some informationfor each not delivered RDMA message and therefore limiting the number ofsupported out-of-order received messages, an unlimited number of notdelivered RDMA messages are supported by storing information on a perTCP hole basis. A “TCP hole” is a term that describes a vacancy createdin the TCP stream as a result of reception of an out-of-order TCPsegment.

Referring to FIG. 12, white blocks indicate missing TCP segments 400that form TCP holes 130A-130C, and shaded/gray blocks 402 indicate acontinuously received TCP stream. Per TCP hole 130A-130C information isstored in connection context 42 (FIG. 2B). A limited number of supportedTCP holes 130A-130C is a characteristic inherited from the TCP protocolimplementation. In particular, the TCP protocol usually limits thenumber of supported TCP holes 130A-130C to, for example, one, two orthree holes. Typically, support of limited number of TCP holes 130A-130Ceffectively means that when an out-of-order TCP segment arrives, openinga new TCP hole, this segment is dropped by TCP logic. FIG. 12illustrates a three-TCP hole implementation. In this case, if a newsegment arrives after the bottom TCP hole 130C, i.e., after the twobottom missing segments 400, this segment will “open” a fourth hole thatis not supported. As a result, that segment would be dropped.

In order to address this situation, the present invention implementstracking of TCP holes 130 (FIG. 12) via connection context 42 (FIGS. 2Aand 2B) rather than tracking of out-of-order messages/segments. Inparticular, as shown in FIG. 2B, the invention stores aPendingReadResponseNum field 300 to count completed RDMA Read Requests,a CompletedSendsNum field 302 to count completed Send messages and aCompletedReadResponseNum field 306 to count completed RDMA ReadResponses. As those skilled in the art should recognize, other fieldsmay be required for each hole, the description of which will not be madefor brevity sake. This approach allows an unlimited number ofout-of-order received RDMA messages waiting for completion and in-orderdelivery. This approach does not limit ability to share a completionqueue 240 (FIGS. 1F-1H) both by receive 212 and send 210 queues withoutany limitation. The details of handling of particular types of messageswill now be described.

First, it should be recognized that delivery of RDMA Write messages 202(FIG. 1G) does not lead to any report to a responder, or anynotification to other hardware logic because of the nature of theoperation. Accordingly, no delivery concerns exist relative to this typeRDMA message.

Second, returning to FIG. 11, with regard to an RDMA Read Responsemessage 206, this operation represents the completion of a pending RDMARead message 204. In this case, storing a CompletedReadResponseNum field306 (FIG. 2B) in connection context 42 that includes a number ofcompleted RDMA Read Response messages 206 per TCP hole 130 is sufficientto provide completion handling logic of the requester with enoughinformation to complete pending RDMA Read work requests 208R. When theTCP hole closes, the number of completed RDMA Read Responses associatedwith this hole is reported to completion handling logic of the requesterto indicate completion of pending RDMA Read work requests 208R.

With regard to RDMA Read Requests, operation of WQE 216RR post includestwo steps: placement of WQE 216RR to Read Queue 414, and a notification,i.e., doorbell ring, to notify RNIC 16 that this WQE can be processed.Placement of WQE 216RR can be done out-of-order. However, as notedabove, the start of the WQE processing (and thus doorbell ring) must becompliant to RDMA ordering rules. That is, the RDMA protocol requiresdelay of processing of inbound RDMA Read messages 204 until allpreviously transmitted RDMA messages of any kind are completed. Thus,the doorbell ring, i.e., notification, should be delayed until allin-order preceding RDMA Read messages 204 are completed. A singledoorbell ring, i.e., notification, can indicate posting of several WQEs216RR.

To resolve the above problem, RNIC 16 according to the invention storesin connection context 42 (PendingReadResponseNum field 300 (FIG. 2B))the number of posted RDMA read response WQEs 216RR waiting for thedoorbell ring (notification) for each TCP hole 130 (FIG. 1B). When a TCPhole 130 is closed, RNIC 16 rings the doorbell (notifies) to confirmposting of PendingReadResponseNum WQEs 216RR to Read Queue 214. Thisindicates that all preceding read messages 204 have been completed, andRNIC 16 can start processing of the posted read response WQEs 216RR.

Referring to FIG. 13, an RDMA Send message 500 represents a uniquesituation. In particular, delivery of a completed Send message includesplacing of a CQE 542 to CQ 540. CQE 542 carries information describingthe completed message (e.g., length, Invalidate STag, etc.). Thisinformation is message specific information, and therefore should bekept for each pending Send message 500. RNIC 16 cannot place a CQE 542before a Send message 500 has been completed (similarly to the placementof RDMA Read Response WQE 508RR in received Read work requests 508R),because a CQ 540 can be shared by several send 510 and receive 512queues, as indicated above.

To resolve this issue without consuming additional RNIC resources, andproviding scalable implementation, OutLogic 40 according to the presentinvention places all information that needs to be included in CQE 542 tothe WQE 516R consumed by that Send message 500. This information is thenretrieved from WQE 516R by verb interface 20 (FIG. 2A) upon aPoll-For-Completion request. RNIC 16 needs to keep the number ofcompleted send messages 500 (in CompletedSendsNum field 302) per TCPhole 130 in connection context 42, which is used to post CQEs 542 to CQ540, when corresponding TCP hole closes. When the TCP hole 130 closes,RNIC 16 places CQEs 542 to CQ 540. The number of CQEs 542 to be placedequals the number of completed Send messages 500 counted for this hole.This approach involves 2N write operations, when N is a number ofcompleted Send messages 500.

One disadvantage of the approach presented above relative to delivery ofan RDMA Send message 500 is that it doubles the number of writeoperations performed by RNIC 16. That is, there is one write to WQE 516Rand one write of CQE 542 for each completed Send message 500. In orderto address this issue, as shown in FIG. 14, according to an alternativeembodiment of the present invention, the content of a CQE 542 is changedto carry a reference counter 544 of WQEs 516R that the particular CQE542 completes. Reference counter 544 is initialized by RNIC 16 to thenumber of Send messages 500 completed for the given TCP hole 130. Verbinterface 20, for each Poll-For-Completion operation, reduces referencecounter 544, and removes CQE 542 from CQ 540 only if the counter becomeszero. In addition, RNIC 16 updates a WQE 516S only if it is holdsgreater than a threshold (M) outstanding Send messages 500 waiting forcompletion. M is a configurable parameter, indicating an amount ofinternal resources allocated to keep information for pending inboundSend messages 500. If M equals zero, then any out-of-order received Sendmessage 500 involves update of WQE 516R (no updated is needed forin-order received Send messages 500).

This embodiment also includes defining two kinds of CQEs 542 andproviding an indicator 546 with a CQE 542 to indicate whether the CQE isone carrying all completion data in the CQE's body, or one that carriespart of completion data with the remainder of the completion informationstored in WQE 516R associated with one or more RDMA Send messages. Thisalternative embodiment reduces the number of write operations to N+1,where N is a number of completed Send messages 500, that were pendingbefore TCP hole 130 was closed.

IV. Conclusion

In the previous discussion, it will be understood that the method stepsare preferably performed by a specific use computer, i.e., finite statemachine, containing specialized hardware for carrying out one or more ofthe functional tasks of the invention. However, the method steps mayalso be performed by a processor, such as a CPU, executing instructionsof a program product stored in memory. It is understood that the variousdevices, modules, mechanisms and systems described herein may berealized in hardware, software, or a combination of hardware andsoftware, and may be compartmentalized other than as shown. They may beimplemented by any type of computer system or other apparatus adaptedfor carrying out the methods described herein. A typical combination ofhardware and software could be a general-purpose computer system with acomputer program that, when loaded and executed, controls the computersystem such that it carries out the methods described herein. Thepresent invention can also be embedded in a computer program product,which comprises all the features enabling the implementation of themethods and functions described herein, and which—when loaded in acomputer system—is able to carry out these methods and functions.Computer program, software program, program, program product, orsoftware, in the present context mean any expression, in any language,code or notation, of a set of instructions intended to cause a systemhaving an information processing capability to perform a particularfunction either directly or after the following: (a) conversion toanother language, code or notation; and/or (b) reproduction in adifferent material form.

While this invention has been described in conjunction with the specificembodiments outlined above, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, the embodiments of the invention as set forth aboveare intended to be illustrative, not limiting. Various changes may bemade without departing from the spirit and scope of the invention asdefined in the following claims. In particular, the described order ofsteps may be changed in certain circumstances or the functions providedby a different set of steps, and not depart from the scope of theinvention.

1. A method of limiting a number of retransmission attempts for a datatransfer via a network interface controller (NIC), the method comprisingthe steps of: storing a number of retransmission attempts for anerror-including data transfer; increasing the number of retransmissionattempts by one each time a retransmission is attempted for the sameerror-including data transfer previously received; and allowing anotherretransmission attempt of the error-including data transfer in the casethat the number of retransmission attempts does not exceed a maximumretransmission attempt number.
 2. The method of claim 1, wherein thestep of allowing includes dropping the data transfer and not confirmingreception of the data transfer.
 3. The method of claim 1, furthercomprising the steps of: storing a largest sequence number among atleast one previously received error-including data transfer and a newlyreceived error-including data transfer; and resetting the number oftransmission attempts in the case that a newly received in-order datatransfer has a sequence number greater than the stored largest sequencenumber.
 4. The method of claim 3, wherein: the newly receivederror-including data transfer is determined to have a sequence numbergreater than the largest sequence number by comparing the sequencenumber of the newly received error-including data transfer to the storedlargest sequence number; and the newly received in-order data transferis determined to have a sequence number greater than the largestsequence number by comparing the sequence number of the newly receivedin-order data transfer to the stored largest sequence number.
 5. Themethod of claim 1, wherein, in the case that the number ofretransmission attempts exceeds the maximum retransmission attemptnumber, the method further comprises the step of placing the datatransfer to a reassembly buffer of the NIC.
 6. The method of claim 5,further comprising the step of sending an Ack for the data transfer. 7.The method of claim 5, further comprising the step of changing a type ofthe data transfer from a first type to a second type.
 8. The method ofclaim 7, further comprising the step of changing the type of the datatransfer from the second type to the first type where: the reassemblybuffer is empty, the data transfer includes aligned DDP segment, and thedata transfer was originated as a first type.
 9. The method of claim 8,wherein the type of the data transfer determines whether the datatransfer is placed to an internal buffer for direct data placement to adestination buffer or to the reassembly buffer.
 10. A system forlimiting a number of retransmission attempts for a data transfer via anetwork interface controller (NIC), the method comprising the steps of:means for storing a number of retransmission attempts for anerror-including data transfer; means for increasing the number ofretransmission attempts by one each time a retransmission is attemptedfor the same error-including data transfer previously received; andmeans for allowing another transmission attempt of the error-includingdata transfer in the case that the number of retransmission attemptsdoes not exceed a maximum retransmission attempt number.
 11. The systemof claim 10, wherein the means for allowing places the data transfer toa reassembly buffer of the NIC where the number of retransmissionattempts exceeds the maximum retransmission attempt number.
 12. Thesystem of claim 10, further comprising: means for storing a largestsequence number among at least one previously received error-includingdata transfer and a newly received error-including data transfer; andmeans for resetting the number of transmission attempts in the case thatthe newly received error-including data transfer has a sequence numbergreater than the stored largest sequence number.
 13. The system of claim12, wherein the newly received error-including data transfer isdetermined to have a sequence number greater than the stored largestsequence number by comparing the sequence number of the newly receivederror-including data transfer to the stored largest sequence number; andthe newly received in-order data transfer is determined to have asequence number greater than the stored largest sequence number bycomparing the sequence number of the newly received in-order datatransfer to the stored largest sequence number.
 14. The system of claim10, further comprising means for changing a type of the data transferfrom a first type to a second type.
 15. The system of claim 14, furthercomprising means for changing changes the type of the data transfer fromthe second type to the first type where: a reassembly buffer of the NICis empty, the data transfer includes aligned DDP segment, and the datatransfer was originated as a first type.
 16. The system of claim 15,wherein the type of the data transfer determines whether the datatransfer is placed to an internal buffer for direct data placement to adestination buffer or to the reassembly buffer.
 17. A computer programproduct comprising a computer useable medium having computer readableprogram code embodied therein for limiting a number of retransmissionattempts for a data transfer via a network interface controller (NIC),the program product comprising: program code configured to store anumber of retransmission attempts for an error-including data transfer;program code configured to increase the number of retransmissionattempts by one each time a retransmission is attempted for the sameerror-including data transfer previously received; and program codeconfigured to allow another retransmission attempt of theerror-including data transfer in the case that the number ofretransmission attempts does not exceed a maximum retransmission attemptnumber.
 18. The program product of claim 17, wherein the allowingprogram code places the data transfer to a reassembly buffer of the NICwhere the number of retransmission attempts exceeds the maximumretransmission attempt number.
 19. The program product of claim 17,further comprising: program code configured to store a largest sequencenumber among at least one previously received error-including datatransfer and a newly received error-including data transfer; and programcode configured to reset the number of transmission attempts in the casethat the new in-order received data transfer has a sequence numbergreater than the stored largest sequence number.
 20. The program productof claim 19, wherein the newly received error-including data transfer isdetermined to have a sequence number greater than the largest sequencenumber by program code configured to compare the sequence number of thenewly received error-including data transfer to the stored largestsequence number.